Computer tomography (CT) provides a diagnosis and measuring method for medicine and testing technology, with the aid of which inner structures of a patient or test object can be examined, without having to carry out operative interventions on the patient or having to damage the testing object. A number of projections from different angles of view are recorded of the object to be examined. A 3D description of the object can be reconstructed from these projections.
A standard method used to solve this problem is presented by the generally known filtered back projection (FBP). This is an analytical method, with which the projections are filtered and back projected into the image region.
One problem for systems of this type is presented by examination objects, which move periodically at least in subareas. An example of this is the illustration of a patient in the area of a moving heart. Projections of a data record are not recorded from the same heart phase, e.g. the diastole, particularly for slow recording modalities, as is generally the case with C-arm systems. During one revolution, the heart can beat a number of times. This results in only one region of directly consecutively recorded projections being associated with the same heart phase. In connection with this region, a large angular gap results, which belongs to other heart phases and thus maps the heart in other movement situations, until projections result again which belong to the desired heart phase. Such projections, which were not recorded in a heart phase to be calculated, generally only differ however in subareas of the image, from those which were recorded during other heart phases. A reconstruction, without accounting for the fact that the individual projections do not belong to the same heart phase, generally results in distorted images of the object and in an increase in the number of artifacts also in object areas which were not moved.
A first, generally known approach (I) to solving this problem consists in omitting projections, which are not present in the desired heart phase and in reconstructing the remaining data record using a FBP reconstruction. The artifacts, which were generated as a result of the projections which were not suited to the heart phase, are herewith reduced, thereby causing artifacts to appear as a result of the missing projections.
A second, generally known approach (II) consists in consecutively recording a number of data records, and herewith ensuring that a complete set of projections is measured at each desired heart phase and that this overall data record is reconstructed by means of an FBP. This is complicated in terms of measurement technology and requires a highly stable heart rhythm.
The publication De Man, Edic, Basu: An iterative algorithm for time-resolved reconstruction of a CT scan of a beating heart. Proc. Eigth Int. Meeting on Fully Three-dimensional Image Reconstruction, SALT LAKE CITY, UTAH, Jul. 6-9, 2005, pp. 356-358, also discloses a third approach (III), in which the reconstruction from data of a revolution is proposed. An iterative reconstruction algorithm is used here, in which projections of all heart phases are used in each iteration cycle. In this way, during reconstruction, less account is taken of those projections which are not associated with the desired heart phase than of those from the desired heart phase. This method is problematical in that artifacts still appear in the reconstructed image data even with a higher iteration number.